The positioning of a lens, mirror or similar optical element (hereafter "lens") involves spatially locating such element in six degrees of freedom. The lens is located translationally relative to each of three orthogonal axes directions generally designated as the x(scan), y(cross-scan), and z(beam path) axes directions. The lens is also located rotationally relative to three rotational directions, generally designated as the .theta..sub.x, .theta..sub.y and .theta..sub.z directions, corresponding to angular rotation, respectively, about each of the x, y and z axes.
Monolithic spherical lenses having one curved surface provide power magnification in two orthogonal directions x and y, and focus parallel rays at a focal point corresponding to the center of curvature of the lens surface. Such lenses are used in laser printers, e.g., for controlling beam spot size, convergence and focusing. Correct positioning of such spherical lenses in the x, y translational and .theta..sub.x, .theta..sub.y rotational directions assures alignment of the focal point and center of the lens relative to an incident beam of light coincident with the z axis. Correct location of the lens along the z axis serves to assure proper focusing of an imaged object. Considerations for locating conjugate and composite spherical lens elements are similar.
Monolithic cylindrical lenses having one curved surface provide magnification in only one direction x or y and focus parallel rays to a line or lens cylinder axis parallel to the other direction y or x. Such lenses are used in laser printers, e.g., for beam shaping, such as for controlling x-direction or y-direction elliptical beam spot size. Cylindrical lenses may be manufactured to have a planar surface opposite the curved surface which is generally parallel to the x-y plane. Such a lens can, thus, be located in the .theta..sub.x and .theta..sub.y rotational directions by orienting the x-y planar surface normal to the incident beam z axis direction. Variations in positioning in the non-magnification direction (i.e. variations in the y direction for magnification in the x direction, and vice versa) are not critical. Thus, once correct orientation of the x-y planar surface is established, locational precision will be needed only in the x or y magnification translational and .theta..sub.z rotational directions. Location in the z direction is left adjustable for focusing purposes.
Traditional mounts for multiple degree of freedom positioning of optical elements nest multiple structural components for independent relative movement, one with respect to the other, to achieve the required translational and/or rotational positioning. U.S. Pat. No. 4,652,095, for example, describes an arrangement of three nested stages, each having a table shiftable along rails in a respective x, y or z translational direction by a threaded rod movable against the force of an opposing spring. The stages are nested, with the optical element mounted for movement with the table of the first stage, the first stage mounted for movement with the table of the second stage, and the second stage mounted for movement with the table of the third stage. U.S. Pat. No. 3,596,863 shows an arrangement of nested flexural pivots, each providing a respective .theta.x, .theta.y, or .theta.z rotational adjustment. Other examples of nested optical element mounting arrangements are given in U.S. Pat. Nos. 3,204,471; 4,077,722; 4,099,852; and 4,655,548.
Mounting arrangements that provide multiple degree of freedom lens positioning, without nesting, are shown in U.S. Pat. Nos. 3,989,358 and 4,408,830. The '358 patent provides independent x and y translational adjustments by micrometer spindles that are moved against knife-edges, displaced 90.degree. circumferentially about a lens retaining ring. The '830 patent provides x, y and x-y translational adjustments by moving inclined faces of screw-driven cradle elements against corresponding angled corners of a rectangular lens retainer.
Conventional devices for achieving six-degree of freedom positioning of optical elements tend to be unduly complex and costly. Moreover, when used for mounting cylindrical lenses in optical systems like those of laser printers or the like, the precise machining utilized to ensure correct positioning in critical directions, is wasted when applied also for non-critical ones. In general, prior art mounts seek to avoid the exertion of any torque directly on the lens itself. See, e.g., U.S. Pat. No. 4,909,599.
Applicant's U.S. patent application Ser. No. 07/765,161, entitled "Adjustable Mount for Cylindrical Lens," filed Sep. 24, 1991, describes an inexpensive lens mount for positioning a cylindrical lens or similar optical element in an optical system like that of a laser printer, wherein six degree of freedom positioning is achieved with a minimum of nesting, taking advantage of physical characteristics of the lens, and employing a push-pull mechanism for applying a biasing torque on the lens, against which adjustments in the x or y axis magnification direction and .theta..sub.z rotational direction are made. The disclosure of that application is incorporated herein and made a part hereof by reference thereto.
In a preferred embodiment of the device described in the '161 application, .theta..sub.x, .theta..sub.y positioning of the lens is achieved by abutting a planar surface, opposite a curved cylindrical surface, of the lens against a corresponding planar surface of an oversized cavity of a lens carrier. The carrier includes planar pads that load up against corresponding planar surfaces of a railed base member of the optical system supporting structure to assure that the cavity planar surface is oriented in the x-y vertical plane. Positioning in the y or x axis non-magnification direction is non-critical and is achieved by other elements in the carrier cavity. Positioning in the x or y axis magnification direction is achieved by fitting planar edge surfaces of the lens in abutment with corresponding planar edge surfaces of a cavity of a lens retaining member, and utilizing a push-pull mechanism to shift the retainer in the magnification direction relative to the lens carrier. The same push-pull mechanism operates to bias the lens clockwise in the .theta..sub.z rotational direction about a pivot point provided in the carrier cavity. Rotational positioning in the .theta..sub.z direction is accomplished by moving a member into the carrier cavity into contact with the lens, to pivot the lens about the same pivot point in opposition to the biasing torque. Means for adjusting the lens in the z axis focus direction in the disclosed embodiment comprises a mechanism for locking the lens carrier in selected position along a rail directed in the z axis direction. Fasteners, threaded through aligned bores of the retainer and carrier, serve to lock the lens after making the x or y and .theta..sub.z adjustments. Those bores are elongated in the x or y magnification direction in the retainer to permit movement of the retainer in that direction relative to the carrier.
The '161 device utilizes a carrier having an internal cavity into which locator pins protrude, and a retainer having an internal cavity that holds the lens, and against which a push-pull mechanism acts to apply a torque to the lens. The push-pull mechanism is an integral part of the mount, so every mount has its own such mechanism. Because the .theta..sub.z position locator pin serves to stabilize the lens during translational movement, the translational x or y magnification direction adjustment is done after the .theta..sub.z angular positioning adjustment is made. The present invention simplifies the structure of the device shown in the '161 application, and applies a push-pull action using a fixture that can be detached for other use on other lens mounts. Moreover, the means for accomplishing rotational .theta..sub.z adjustment is made independent of the means for accomplishing translational x or y adjustment, so rotational and translational adjustments can be made in any order.